Dynamic recrystallization behavior of a typical nickel-based superalloy during hot deformation

The dynamic recrystallization (DRX) behavior of a typical nickel-based superalloy is investigated by the hot compression tests. Based on the conventional DRX kinetics model, the volume fractions of DRX are firstly estimated. Results show that there is an obvious deviation between the experimental and predicted volume fractions of DRX when the forming temperature is below 980 °C, which is induced by the slow dynamic recrystallization rate under low forming temperatures. Therefore, the segmented models are proposed to describe the kinetics of DRX for the studied superalloy. Comparisons between the experimental and predicted results indicate that the proposed segmented models can give an accurate and precise estimation of the volume fractions of DRX for the studied superalloy. In addition, the optical observation of the deformed microstructure confirms that the dynamically recrystallized grain size can be well characterized by a power function of Zener–Hollumon parameter.     

Effect of strain rate on microstructure evolution of a nickel-based superalloy during hot deformation

The hot deformation behavior of a nickel-based superalloy was investigated by means of isothermal compression tests in the strain rate range of 0.001–10 s⁻¹ at 1110 °C. Transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technique were used to study the effect of strain rate on the microstructure evolution of the alloy during hot deformation. The results revealed that the dynamic recrystallization (DRX) process was stimulated at high strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$) due to the high dislocation density and adiabatic temperature rise. Meanwhile, high nucleation of DRX and low grain growth led to the fine DRX grains. In the strain rate rage of 0.001–1 s⁻¹, the volume fraction of DRX grains increased with the decreasing strain rate, and the grain growth gradually governed the DRX process. Moreover, the strain rate has an important effect on DDRX and CDRX during hot deformation. On the other hand, particular attention was also paid to the evolution of twin boundaries during hot deformation. It was found that there was a lower fraction of Σ3 boundaries at the intermediate strain rate of 1 s⁻¹, while the fractions of Σ3 boundaries were much higher at both the lower strain rates ($\dot{\epsilon }⩽0.1{\text{s}}^{-1}$) and higher strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$).     

Effect of boron addition on the microstructure and mechanical properties of K4750 nickel-based superalloy

The effect of boron addition at 0,0.007 wt.% and 0.010 wt.% on the microstructure and mechanical properties of K4750 nickel-based superalloy was studied.The microstructure of the as-cast and heat-treated alloys was analyzed by SEM,EPMA,SIMS and TEM.Lamellar M₅ B₃-type borides were observed in boroncontaining as-cast alloys.After the full heat treatment,boron atoms released from the decomposition of M₅ B₃ borides were segregated at grain boundaries,which inhibited the growth and agglomeration of M₂₃C₆ carbides.Therefore,the M₂₃C₆ carbides along grain boundaries were granular in boron-containing alloys,while those were continuous in boron-free alloys.The mechanical prope rty analysis indicated that the addition of bo ron significantly improved the tensile ductility at room tempe rature and stress rupture properties at 750℃/430 MPa of K4750 alloy.The low tensile ductility at room temperature of 0 B alloy was attributed to continuous M₂₃C₆ carbides leaded to stress concentration,which provided a favorable location for crack nucleation and propagation.The improvement of the stress rupture properties of boron-containing alloys was the result of the combination of boron segregation increased the cohesion of grain boundaries and granular M₂₃C₆ carbides suppressed the link-up and extension of micro-cracks.     

Fabrication of dense thin sheets of γ-TiAl by hot isostatic pressing of tape-cast monotapes

A method is described for the fabrication of dense thin sheets of γ titanium aluminide (γ-TiAl) by a powder metallurgy route involving hot isostatic pressing (HIP) of tape-cast monotapes. Gamma-TiAl powder (particle size <90 μm) was incorporated into a concentrated slurry by mixing with an organic binder in a solvent and the system was tape-cast to form sheets with a thickness of 400–600 μm. After insertion of the tape-cast sheet into a HIP can and binder removal in situ by thermal decomposition, HIP at 1100 °C under a pressure of 130 MPa produced dense sheets with a thickness of 250–400 μm. The free, dense sheets with a fine-grain microstructure were obtained by dissolution and oxidation of the HIP can. The carbon content of the fabricated sheets was 0.035 wt.%. Facile adaptation of the process to the production of γ-TiAl thin sheets with complex shapes is expected.     

热等静压技术在镍基铸造高温合金领域的应用研究

镍基铸造高温合金是航空发动机与燃气轮机生产制造过程中应用的主要材料之一,在航空航天、能源工业、船舶舰艇等领域有着广泛的应用。现代航空工业的飞速发展离不开高温合金综合性能的快速提升,而热等静压技术在镍基铸造高温合金领域的应用对镍基铸造高温合金综合性能的改进方面发挥了举足轻重的作用。本文介绍了热等静压技术的工作原理与应用发展历史,总结了热等静压技术在镍基铸造高温合金领域的研究应用现状,重点阐述了热等静压技术对铸造高温合金的致密化作用机理与组织性能影响、热等静压对长期服役镍基铸造高温合金组织修复研究以及实现两种镍基高温合金扩散连接的应用优势与研究成果。同时指出热等静压技术研究中存在的一些问题及国内热等静压技术在镍基铸造高温合金领域的发展趋势。     

Improvement of optical properties of Nd:YAG transparent ceramics by post-annealing and post hot isostatic pressing

The Nd:YAG transparent ceramics were fabricated by vacuum sintering. The Nd:YAG samples were annealed at 1450 °C for 20 h in air and followed by hot isostatic pressing (HIP) at 1700 °C for 2 h in 200 MPa Ar and then re-annealed at 1250–1450 °C for 10 h in air. The experimental results showed that the optical properties of Nd:YAG samples varied markedly with different post treatments. After air annealing at 1450 °C for 20 h and HIP at 1700 °C for 2 h under 200 MPa of Ar and then air re-annealing at 1250 °C for 10 h, the transmittances of the samples increased from 51.2% to 77.2% (at 400 nm) and 78.4% to 83.6% (at 1064 nm), respectively. The annealing and HIP are effective post treatments to reduce oxygen vacancies and intergranular pores respectively in Nd:YAG transparent ceramics.     

Enhanced tensile properties in a Mg-6Gd-3Y-0.5Zr alloy due to hot isostatic pressing (HIP)

Hot isostatic pressing (HIP) was applied to Mg-6Gd-3Y-0.5Zr (GW63) alloy to reduce shrinkage porosity, thus, to enhance the integrity and reliability of castings. During HIP process, shrinkage porosity was closed by grain compatible deformation and subsequent diffusion across the bonding interface. The amount of initial shrinkage porosity was the key factor for shrinkage porosity closure. HIP was testified to be effective on shrinkage porosity reduction in GW63 alloy due to its relatively narrow solidification range and resultant low content of initial shrinkage porosity in most sections, leading to higher tensile properties both in as-cast and cast-T6 condition. The improvement in tensile properties was mainly because of shrinkage porosity reduction and resultant effective rare-earth (RE) elements homogenization and precipitation strengthening.     

Stability of microstructure and mechanical properties of K17G cast nickel-base superalloy during prolonged aging

K17G cast nickel-base superalloy has an attractive combination of mechanical properties: high strength and good ductility as well as low density. In particular, after prolonged exposure at 750, 850 (both up to 10000 h) and 950°C (up to 1500 h), this alloy proved to have excellent phase stability and no sigma phase was found. The effects ofprolonged exposure on high temperature tensile and stress rupture properties were not serious and decreased gradually with aging time. The change in properties of specimens taken directly from the turbine blades followed those ofthe cast-to-size specimens. The rupture lives, tensile and rupture ductilities were approximately one third lower compared with those ofthe cast-to-size specimens, but still remained at reasonably high levels.     

Development of Ni-base metal matrix composites by powder metallurgy hot isostatic pressing for space applications

In this work, near-net-shape powder metallurgy hot isostatic pressing (NNS PM HIP) of Ni-base metal matrix composite (Ni-MMC) was developed to improve the hardness and wear properties of turbopumps mechanical seals. Silicon carbide (SiC) and titanium diboride (TiB₂) fine powders were used as reinforcements with different ratios to improve the hardness and consequently the tribological properties of the developed Ni-MMC material. Powder characterisation was performed on the blended powders to check the homogeneity of the mixed powders. The hot isostatically pressed (HIPed) Ni-MMC microstructures were analysed using scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. The HIPed material showed a fully dense microstructure with a continuous network of ceramic reinforcement particles at the prior particle boundaries (PPBs). Furthermore, microhardness tests were performed on IN625, IN625-SiC and IN625-TiB₂ to understand the impact of the reinforcement on the microhardness. It was demonstrated that the volume percentage of ceramic reinforcement in the IN625 matrix plays a crucial role in achieving higher hardness by increasing the fraction of hard phases appearing in the microstructure of the developed Ni-MMC material. The final part of the work focuses on the canister design and manufacture of a near-net-shape (NNS) mechanical gas seal using IN625 based MMC to demonstrate the feasibility of manufacturing mechanical seals through the NNS PM HIP technique. Overall, IN625 based MMCs resulted in a drastic improvement in tribological properties if compared to the base material. Furthermore, the employment of the PM HIP consolidation technique resulted in a fully dense and homogeneous microstructure, highlighting the potentials of PM HIP in the generation of novel composite materials.     

Superlattice stacking fault formation and twinning during creep in γ/γ′ single crystal superalloy CMSX-4

This paper describes further insight which has been gained into the formation of deformation twins during high temperature creep in a high volume fraction single crystal superalloy (CMSX-4) and correlation with the nature of superlattice stacking faults also observed. In general it is found that the formation of high temperature twinning can always be associated with the loading orientation in which SESF formation is expected from a determination of the sign of the shear stress. One can rationalise the reason for this twinning being associated with extrinsic stacking faults, by consideration of the critical radius of stacking fault loop nucleation. In particular calculations at 1223 K demonstrate a minimum in radius being associated with four overlapping faulted planes for the extrinsic case. Although the process by which twin formation occurs has not been observed, it is shown that twin formation can take place by the passage of extrinsic stacking faults within the precipitate which operate on every alternate plane of the structure. If one considers formation to occur via a pole mechanism similar to that in a face centred cubic structure a mechanism for this occurrence is postulated due to the fact that climb of the helix structure as it rotates around a pair of suitable matrix dislocations, will amount to double that which occurs in the single pole case.     

Interaction of hot isostatic pressing temperature and hydrostatic pressure on the healing of creep cavities in a nickel-based superalloy

The interaction of soaking temperature and hydrostatic pressure on the evolution of creep cavities under hot isostatic pressing (HIP) for a nickel-based superalloy is investigated. The concentrically-oriented γ′ rafting structure occurs around the cylindrical creep cavity under 1453 K/150 MPa/1 h. The cylindrical cavity also tends to break into a row of spherical voids due to the classical Rayleigh instability. HIP temperature and pressure, acting as two driving factors for solute diffusion, interact and influence the healing behavior of creep cavities.     

Effect of ultrasonic micro-forging treatment on microstructure and mechanical properties of GH3039 superalloy processed by directed energy deposition

In this work,ultrasonic micro-forging treatment(UMFT)was introduced to achieve homogeneous microstructure,reduce defects and improve mechanical properties of GH3039 superalloy cladding layer processed by directed energy deposition(DED).The microstructure,defects and mechanical properties of the cladding layers treated by UMFT with different ultrasonic powers(UIPs)were investigated.Results revealed a gradient structure as equiaxed grains distributed at the top,a columnar-to-equiaxed transition(CET)region that mixed of columnar dendrites and equiaxed grains distributed at the middle and colum-nar dendrites at the bottom of the cladding layer was formed.After UMFT,the proportion of equiaxed grains was increased,the average size of equiaxed grains was refined to 10 μm from 16 μm,the orien-tation of grains was more uniform and the phases enriched of Al,Ti,C,Nb and Mo were precipitated.The grain refinement can be attributed to the fracture of columnar dendrites induced by the ultrasonic vibration during solidification.Besides,the porosity of the cladding layer was reduced after UMFT.The microhardness of the cladding layers exhibited a depth-dependent gradient at the top region.The micro-hardness of the top surface was the highest and showed an increasing trend with the increase of UIP.The microhardness of different grain morphologies exhibited no substantial difference.However,due to grain refinement and precipitation of strengthening phase induced by UMFT,the microhadness of some local locations were improved.These results indicated UMFT has a significant effect on improving the microstructure,defects and mechanical properties of the deposited cladding layer.     

Effect of Hot Isostatic Pressing Conditions and Cooling Rate on Microstructure and Properties of Ti-6Al-4V Alloy from Atomized Powder

The effects of temperature and pressure on density, microstructure and mechanical properties of powder compacts during hot isostatic pressing（HIPping） were investigated. Optimized HIPping parameters of temperature range from 900 to 940℃, pressure over 100 MPa and holding time of 3 h, were obtained. Tensile properties after different heat treatments show that both the geometry of samples and cooling rate have a significant influence on mechanical properties. Finite element method was used to predict the temperature field distribution during HIPped sample cooling, and the experimental results are in agreement with simulation prediction. The interaction of HIPping parameters was analyzed based on the response surface methodology（RSM） in this study.     

Effect of cooling rate on microstructure,microsegregation and mechanical properties of cast Ni-based superalloy K417G

The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900?C and stress rupture test was performed at 950?C with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ'phase and the size of γ/γ'eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ' eutectic decreased.In the cooling rate range of 1.42?C s~(-1)–0.84?C s~(-1),the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ' eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84?C s~(-1) showed the best comprehensive mechanical properties.     

Effects of surface modification of β-CaSiO3 on electrospun poly(butylene succinate)/β-CaSiO3 composite fibers

In this experiment, pure PBSU fibers, PBSU/12.5% β-CaSiO₃, and PBSU/25% β-CaSiO₃ composite fibers were fabricated by electrospinning. In order to investigate the effects of surface modification of β-CaSiO₃ on composite fibers, β-CaSiO₃ nanowires were surface esterified using dodecyl alcohol. SEM micrographs showed that composite materials with modified β-CaSiO₃ have homogeneous fibrous structures similar as that of pure PBSU fibers, while the fibers containing unmodified β-CaSiO₃ were inhomogeneous and much larger in diameter, and also junctions where β-CaSiO₃ agglomerated could be found. Mechanical testing showed that with the addition of unmodified β-CaSiO₃ into PBSU matrix, the tensile strength of fibrous materials decreased obviously, and the decrease degree increased with increased β-CaSiO₃ content. However, the tensile stresses of composite materials after surface modification of β-CaSiO₃ turned back and increased about 40% compared to those containing unmodified β-CaSiO₃. All of these results suggested surface modification of β-CaSiO₃ was an effective approach to obtain composite fibrous materials with better morphologies and enhanced mechanical properties, and this method is supposed to be feasible in other fibrous material systems.     

Study on the castability and mechanical properties of thin-walled nickel-base superalloy

The castability and mechanical properties of thin-walled nickel-base superalloy castings fabricated by gravity casting and centrifugal casting were investigated. It is shown that, despite its microporosity has slight change, the casting fabricated by centrifugal casting presents fewer misruns and less Laves phase than that of gravity casting. The ultimate tensile strength, yield strength and stress rupture lives of centrifugal casting are improved by about 2.2, 7.4 and 41.1%, respectively. However, the elongation (EL) and reduction in area are sharply decreased by about 10.9 and 25.0%, respectively. Paradoxically, the stress ELs of both castings are similar. The seemingly contradictory results in mechanical properties stem from the opposite effects of centrifugal force on hardening elements macrosegregation and melt turbulence.     

Investigations of MX and γ′/γ″ precipitates in the nickel-based superalloy 718 produced by electron beam melting

Samples with a composition similar to the nickel-based superalloy Inconel alloy 718 were produced by electron beam melting of prealloyed powder and investigated with respect to type and composition of the strengthening precipitates. The matrix consists of γ grains orientated in nearly the same direction, almost like a single crystal. Coarse precipitates (<2 μm), mostly of the (Ti,Nb)(C,N,B) type with B1 structure, are aligned along the growth direction. TEM and APFIM investigations of the γ matrix revealed very fine γ″ precipitates of around 5–10 nm in size. Additionally, at small angle grain boundaries, coarser γ″ precipitates of 50–100 nm in size have been observed. The 0 01 γ//0 0 1 γ″ and {1 0 0} γ//{1 0 0} γ″ orientation relationship between γ and γ″, known from literature [M. Sundararaman, P. Mukhopadhyay, Mater. Charact. 31 (1993) 191–196], was confirmed. Some γ′ precipitates of 2–5 nm in size were observed by means of FIM.     

Effect of high-temperature hot corrosion on the low cycle fatigue behavior of a directionally solidified nickel-base superalloy

The low cycle fatigue behavior of a directionally solidified nickel-based superalloy DZ125 was examined at 850 °C in air using bare and salt-coated specimens. Experimental results show that the salt-coated specimen showed relatively low fatigue life compared with the bare specimen, and this effect accelerated with the increased applied maximum stress. Damage of hot corrosion in fatigue life was found to be associated with the reduction of the bare area and the early crack initiation from the weaken grain boundaries of recrystallized grains.     

Effect of Phosphorus on Microstructure and High Temperature Properties of a Cast Ni—base Superalloy

Effect of phosphorus on the microstructure and high temperature properties of a cast Ni-base superalloy M963 has been investigated.SEM observation and EDS analysis showed that P was mostly enriched in the interdendritic region,and the P-rich phase was formed in the front position of finally solidified eutectics in high P doped alloys.It was found that the P-rich phase,as preferred initiation and propagation site of cracks,could aggravate the fracture process at high temperature in high P doped alloys.Consequently,high P addition would reduce remarkably the ductility and creep life of M963 superalloy at high temperature.     

碳对镍基高温合金AM3热腐蚀性能的影响

为了更充分地了解碳对镍基高温合金热腐蚀性能的影响,提高合金的耐热腐蚀性能,本文研究了不同碳含量镍基高温合金AM3在850℃条件下,经75%Na2SO4+25%NaCl饱和混合盐溶液热腐蚀5 h的行为。利用扫描电镜(SEM)、X射线衍射(XRD)等测试方法分析了合金热腐蚀后的组织形貌和腐蚀产物。研究表明,腐蚀5 h过程中,碳含量(质量分数)为0、0.045%、0.15%的合金持续增重,碳含量为0.085%的合金在2~4 h发生氧化膜与盐膜的碱性熔融,有失重现象存在。合金加入碳后,促使合金表层腐蚀层变薄且与基体结合力变好。含碳的镍基高温合金腐蚀层产物以氧化物为主,主要有NiO、TiO2、Al2O3、Cr2O3。碳含量(质量分数)为0.085%、0.15%的合金由于腐蚀层氧化物较薄,可检测到Al4CrNi15、Ni3Al相的存在。综合分析发现碳含量为0.085%时,合金耐热腐蚀性能达到最优。